X-ray tube



April 5, 1938. c. M. SLACK 2,113,334

X-RAY TUBE Filed Oct. 30, 1954 INVENTOR A C? M SZflC/f 277 62 ATTORNEY Patented Apr. 5, 1938 UNITED STATES PATENT OFFICE X-RAY TUBE Application October 30, 1934, Serial No. 750,633

4 Claims.

This invention relates to electric discharge tubes and more particularly to such tubes which operate at high voltages and with pure electron discharge, such for example, as X-ray tubes.

5 Tubes of this type usually consist of a vitreous evacuated envelope provided with oppositely disposed electrodes sealed therein which comprise an anode and a thermionic cathode. Upon heating of the thermionic cathode to incandescence and the application of a high potential between the anode and cathode the electrons emanating from the cathode impinge upon the anode and cause the generation of X-rays. During operation of the tube considerable scattered electrons is which may be secondary, reflected, or stray electrons, bombard the glass walls of the envelope frequently resulting in puncture thereof or these electrons travel along the envelope adjacent the anode stem destroying the seal between the envelope and anode rendering the tube useless.

A further disadvantage frequently present in tubes of this general character resides in the formation of undesirable fluorescence about the anode stem during operation. The most common cause of this fluorescence is due to the bombarding electrons liberating gas molecules from the glass upon which they impinge by mechanical impacts, or by molecular dissociation, thus causing the tube to frequently acquire a slight gas pressure with continued use, which causes the user to believe his tube has become gassy thus destroying the high vacuum, and necessitating frequent renewals of the tube on the part of the manufacturer.

It is accordingly an object of my present invention to provide an electric discharge tube wherein the scattered electrons are intercepted and prevented from traveling along the tube in the vicinity of the seal.

Another object of my present invention is the provision of an electric discharge device wherein the evacuated envelope is so constructed that scattered electrons are intercepted and an attendant electrostatic field is generated which 45 confines any incidental fluorescence occurring during operation of the tube substantially to the discharge zone between the electrodes.

Another object of my present invention is the provision of an X-ray tube wherein the envelope 50 is provided with a portion in juxtaposition with at least one of the electrodes for the purpose of intercepting scattered electrons and generating an electrostatic field having a repelling force, for the purpose of preventing electrons from traveling along the envelope and to confine any incidental fluorescence which might occur during operation of the tube substantially to the discharge zone between the anode and cathode.

A further object of my present invention is the provision of an X-ray tube wherein the evacu- 5 ated envelope is provided with a sleeve portion extending interiorly of the envelope a substantial distance beyond the points of seal of the respective electrodes thereto, and which has a constricted portion in the vicinity of at least one -10 of the electrodes, for the purpose of intercepting scattered electrons to prevent the same from traveling along the envelope and deleteriously affecting the seal, and for generating an electrostatic field having a repelling force which confines any incidental fluorescence'occurring during operation of the tube substantially to the discharge zone between the anode and cathode.

Still further objects of my present invention will become obvious to those skilled in the art by reference to the accompanying drawing wherein:

Figure l is a view of an X-ray tube constructed in accordance with my present invention partly in section and with parts broken away to better illustrate the construction;

Fig. 2 is a sectional view taken on the line II-II of Fig. 1;

Fig. 3 is a sectional View taken on the line III-III of Fig. 1, and

Fig. 4 is a modification of an X-ray tube constructed in accordance with my present invention.

Referring now to the drawing in detail I have shown in Fig. 1 a substantially cylindrical evacuated envelope 5 which may be of uniform diam- 5 eter or have a portion thereof in the vicinity of the cathode electrode of slightly less diameter for the conservation of space in actual use of the tube. The envelope is provided at one end thereof with a reentrant sleeve portion 6 to which is sealed, in a manner well known to the art, a cathode electrode 1. This electrode is provided with a metallic focusing cup 8 having recessed therein a filamentary cathode 9 in accordance with the usual construction.

The thermionic cathode 9 is arrangedto receive heating current from a suitable source'of supply through a pair of conductors In and i2 as well as being connected to one side of a high voltage source of supply. The opposite end of 50 the envelope 5 is similarly provided with a reentrant sleeve l3 to which is appropriately sealed an anode electrode l4 provided with a refractory metal target IS. The anode l4 may be supplied with a potential from a suitable source by means of a conductor I 6 and the same may be cooled during operation of the tube by the circulation of a cooling medium interiorly of the anode through the medium of concentric conduits l1 and I8, if desired. Upon incandescence of the thermionic cathode and the application of a high potential between the electrodes I and I4 electrons will flow from the cathode and bombard the target IS with the accompanying generation of X-rays.

In order to prevent any scattered electrons, such as reflected or secondary electrons, from the anode from impinging upon the glass walls of the envelope, I provide the envelope with a sleeve l9 which forms an integral part thereof by being secured to the inner walls of the envelope at a point behind the seal of the anode to the reentrant sleeve [3. This sleeve extends interiorly of the envelope a substantial distance beyond the point of seal of the cathode electrode 1 with the reentrant sleeve portion 6 and is open at one end in order that the same may be evacuated during evacuation of the entire envelope.

This sleeve is provided with a constricted portion 23 in the vicinity of the anode l4 thus disposing the same in juxtaposition with the electrode at a point rearwardly of the anode head carrying the target 15. This constriction 20 extends longitudinally of the anode electrode a substantial distance, as shown in Fig. l, and is spaced relative to the electrode M a sufficient distance to prevent an accumulation of sputtered metal particles with an attendant building up of a charge upon the sleeve l9 at the working voltage range of the X-ray tube, yet this spacing is confined to a suflicient minimum as to prevent the scattered'electrons from traveling along the electrode M. In the actual construction of a tube in accordance with my present invention I found that for a tube having a rating of 220 kv. the spacing of the constricted portion 20 with respect to the anode should preferably be from to inch and its length approximately 1 to 2 inches.

During operation of the tube at 220 kv. and slightly higher the constricted portion 20 due to its length and spacing not only intercepts scattered electrons, thus preventing the building up of an electrostatic charge upon the outer walls of the envelope, but in addition causes the generation of an electrostatic field about the anode electrode l 5, which has a repelling force preventing the scattered electrons from traveling along the envelope to the vicinity of the point of seal of the anode with the envelope.

A similarly constricted portion 22 of the sleeve I9 surrounds the cathode electrode 1 and is accordingly in juxtaposition therewith thus disposing the greater diameter portion of the sleeve IS in the discharge zone substantially between the ends of the oppositely disposed electrodes 1 and M. The close proximity of the sleeve portion 22 to the cathode electrode 1 likewise causes the generation of an electrostatic field about the cathode having a repelling force which prevents cold emission, that is, the emanation of electrons from the metallic focusing cup 8 or other metal parts of the cathode. The spacing of the constriction 22 relative to the cathode as well as the depth the latter is recessed in the constriction, as shown in Fig. l, to prevent cold emission is dependent upon the working voltage range of the X-ray tube. The higher the Working voltage range of the tube the greater the recessing of the cathode within the constriction 22 required to prevent cold emission, and the spacing of the constriction relative to the cathode is diminished as the working raneg increases.

Inasmuch as the electrostatic field about the anode It prevents the scattered electrons from traveling along the anode any incidental fluorescence occurring during operation of the tube is likewise confined substantially to the discharge zone comprising the greatest diameter portion of the sleeve I9.

The electrostatic field about both the anode and cathode accordingly confines the electron stream more or less to a restricted path flowing in the general direction of the target l5. In a tube constructed in this manner there is an entire absence of an electrostatic charge upon the outer glass walls of the envelope due to the interception of scattered electrons by the sleeve I9.

Also due to the action of the constricted portions 20 and 22 of the sleeve l9 cold emission from the cathode is prevented as well as scattered electrons traveling along the envelope deleteriously affecting the anode seal. Due to a more or less confining of the scattered electrons there is an entire absence of any tendency for the tube to become gassy during operation and any incidental fluorescence which might occur is confined substantially to the discharge zone between the anode and cathode. As a still further precaution for protection of the anodeenvelope seal if desired, the anode may be provided with a skirted sleeve 23 extending over the point of seal having its open free end disposed in a direction away from that of the electron flow, although such sleeve is not essential provided the length and spacing of the constriction 20 is properly proportioned.

In Fig. 4 I have shown a modification of the structure shown in Fig. 1 wherein the only differences in construction reside in the elimination of the inner sleeve H), as described in the preceding modification, and in lieu thereof I provide the envelope 5 with an annular constricted portion 24 of lesser diameter than the remainder of the tubular envelope. This portion 24 is, however, disposed in juxtaposition to the anode electrode 14 in substantially the same manner as the constricted portion 20 of the sleeve IS with the length and spacing thereof being substantially the same as described relative to Fig. 1.

While this construction accomplishes the same purpose as just described relative to Fig. 1 in that scattered electrons are precluded from traveling along the walls of the envelope due to the repelling force of the electrostatic field thus preventing deleterious effects to the anode-envelope seal, an electrostatic charge would in this modification accumulate upon the walls of the envelope 5.

- Accordingly in order to prevent puncture of the envelope it is necessary that the same be constructed of greater wall thickness, as shown at 25, than the envelope of Fig. 1. Also there is no concentration of an electrostatic field about the cathode such as would preclude cold emission as in the preferred embodiment shown in Fig. 1.

It thus becomes obvious to those skilled in the art that I have provided an electron discharge tube wherein secondary, reflected, or stray electrons, are prevented from traveling the length of the glass tube and deleteriously afiecting the anode seal and any fluorescence attendant operation of the tube is confined substantially to the section of the glass envelope substantially within the discharge zone between the anode and cathode. Moreover, cold emission from the cathode is substantially prevented and the life of the tube as a whole is greatly increased as well as the economy of manufacture.

Although I have shown and described several embodiments of my invention I do not desire to be limited thereto as various other modifications of the same may be made without departing from the spirit and scope of the appended claims.

What is claimed:

1. An electrical discharge tube comprising an evacuated envelope provided with electrodes therein including an anode and a thermionic cathode extending longitudinally of said envelope with their inner extremities diametrically disposed with respect to each other, said envelope being provided with a sleeve portion sealed at one end to said envelope and disposed interiorly thereof and extending longitudinally of said envelope with the opposite end of said sleeve portion terminating a substantial distance beyond the inner extremity of said anode and cathode, and said sleeve portion having an abruptly formed constriction spaced in close proximity to said anode and extending a substantial distance longitudinally thereof for intercepting scattered electrons and generating an electrostatic field having a repelling force for preventing electrons from traveling along said envelope in the vicinity of said anode and to confine any incidental fluorescence which might occur during operation of said tube substantially to the discharge zone between said anode and cathode.

2. An electrical discharge device comprising an evacuated envelope provided with electrodes therein including an anode and a thermionic cathode extending longitudinally of said envelope with their inner extremities diametrically disposed with respect to each other, said envelope being provided with a sleeve portion sealed at one end to said envelope and disposed interiorly thereof and extending longitudinally of said envelope with the opposite end of said sleeve portion terminating a substantial distance beyond the inner extremity of said anode and cathode to prevent an electrostatic charge from accumulating on the walls of said envelope, and said sleeve portion having an abruptly formed constriction in juxtaposition to said cathode and extending a substantial distance longitudinally thereof for generating an electrostatic field about said cathode having a repelling force suflicient to prevent cold electron emission from said cathode.

3. An electrical discharge device comprising a tubular evacuated envelope provided with electrodes sealed therein including an anode and a cathode extending longitudinally of said envelope with their inner extremities diametrically disposed with respect to each other, said envelope being provided with a sleeve portion sealed thereto and disposed interiorly of said envelope and extending longitudinally thereof beyond the point of seal of both said electrodes to said envelope, and said sleeve portion having an abruptly formed constriction between the point of seal of said anode to said envelope and the discharge zone between said anode and cathode and between the point of seal of said cathode with said envelope and the discharge zone between said anode and cathode, for generating an electrostatic field about said cathode having a repelling force to prevent cold electron emission therefrom, and for generating a similar field about said anode for the purpose of preventing electrons from travelling along said envelope in the vicinity of said anode and to confine any incidental fluorescence occurring during operation of said tube substantially to the discharge zone between said anode and cathode.

4. An electrical discharge tube comprising a tubular evacuated envelope provided with electrodes sealed therein including an anode and a thermionic cathode extending longitudinally of said envelope with their inner extremities diametrically disposed with respect to each other, said envelope being provided with a sleeve portion disposed interiorly thereof having an abruptly formed annular portion of reduced diameter adjacent one of said electrodes and disposed in juxtaposition thereto between the point of seal of said electrode with said envelope and the extremity thereof, for the purpose of intercepting scattered electrons to prevent the same from traveling along said envelope and to confine any incidental fluorescence which may occur during operation'of said tube substantially to the discharge zone between said anode and cathode.

CHARLES MORSE SLACK. 

